Polymer electrolyte fuel cells using a liquid organic compound, such as methanol, ethanol, or dimethyl ether, for fuel are characterized by low noise, low operating temperature (about 70 to 80° C.), and ease of fuel supply. Therefore, the polymer electrolyte fuel cells are expected to be applied widely to a portable electric power generator, a power source for electric automobiles, and a power source for light vehicles, such as an electric motorcycle, an electric bicycle, a wheelchair, or mobility scooter.
For these applications, a direct methanol fuel cell (hereinafter referred to as a DMFC) using methanol for fuel has advantages that a fuel processor can be omitted, that fuel can be supplied at room temperature, that fuel cost to the output is cheap as compared to gasoline, and that electricity can be generated at a low temperature of 50 to 60° C. to reduce a startup time. In particular, an “active” type DMFC in which the fuel is forced to circulate by a pump can provide a high output power of several tens of watts (W) to several hundreds of watts (W). Thus, the active DMFC is suitable for supplying power to a relatively low power device, such as an electronic device or a lighting apparatus. By increasing the cell size or the number of stacked cells, the DMFC of 1 kW or more can be provided and applied to mobile objects.
The DMFC is controlled such that the concentration of methanol is in a predetermined range. The DMFC is provided with a sensor for measuring the concentration. The reason why the concentration of methanol needs to be controlled is that the amount of methanol penetrating an electrolyte membrane is increased as the methanol concentration becomes high, which results in a decrease in cell voltage or output. Another method for controlling the methanol concentration is proposed which estimates the methanol concentration from a state of a fuel cell without using any sensor.
Other methods in related arts for controlling the methanol concentration are disclosed, for example, in Japanese Unexamined Patent Application Publication No. 2007-95679, Japanese Unexamined Patent Application Publication No. 2005-332597, Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2004-537150, Japanese Unexamined Patent Application Publication No. 2006-73486, Japanese Unexamined Patent Application Publication No. 2007-294334, Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2004-527067, and Japanese Unexamined Patent Application Publication No. 2004-327354.
Japanese Unexamined Patent Application Publication No. 2007-95679 discloses a technique for calculating the methanol concentration from a concentration-voltage curve and a variation range of voltage at the time of methanol supply.
Japanese Unexamined Patent Application Publication No. 2005-332597 discloses a concentration adjustment device for a direct methanol fuel cell that quantitatively calculates the methanol concentration in an aqueous methanol solution by utilizing the principle that the amount of heat of dissolution changes depending on the methanol content.
Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2004-537150 discloses a method for controlling the methanol concentration in a DMFC without a concentration sensor. In this method, the control process is performed by sampling current-voltage characteristic curve of a fuel cell using small fluctuations of a system variable current and a methanol concentration.
Japanese Unexamined Patent Application Publication No. 2006-73486 discloses a method for adjusting the concentration of diluted fuel based on the relationship between a flow rate of the fuel and an output voltage of an electricity generating portion of the fuel cell. The method involves increasing and decreasing the flow rate of the diluted fuel supplied to the electricity generating portion, and then measuring the output voltage of the electricity generating portion.
Japanese Unexamined Patent Application Publication No. 2007-294334 discloses a technique which involves filling one common cell in a fuel cell stack with fuel, and calculating the concentration of the fuel based on the amount of generated electricity in the common cell.
Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2004-527067 discloses a technique for calculating the methanol concentration from the potential at an anode located near the end of a methanol passage. The methanol concentration becomes lowest at the end of the methanol passage. The potential near the anode is highly sensitive to changes in methanol concentration. The technique, using such characteristics, compares the detected potential with a predetermined reference potential or a potential in another position of the anode, and uses a difference between the potentials to adjust the methanol concentration.
Japanese Unexamined Patent Application Publication No. 2004-327354 discloses an operating method of a fuel cell for adjusting the concentration of fuel to the optimum level by checking an output density at an electricity generating portion of a DMFC based on the relationship between the concentration of fuel and the output density.
It is an object of the present invention to provide an operation control method which estimates the methanol concentration, while omitting a methanol concentration sensor. Another object of the present invention is to provide a method that can adjust or correct the optimum methanol concentration based on a required output even if the performance of a fuel cell is reduced over time, suppressing the reduction in performance of the fuel cell.
The above objects will be more specifically described below. The invention will simultaneously solve the following three technical problems. The technical problems will be described focusing on the oxidation and removal of methanol contained in exhaust gas of each of fuel and an oxidant. Formaldehyde and formic acid can be oxidized and removed in the same way. Thus, the invention can be applied to fuel cells using other liquid organic fuels, such as ethanol.
The first problem to be solved by the present invention is to determine whether the concentration of fuel supplied to the fuel cell is optimum or not even when the temperature of the fuel cell or the outside air changes. The first problem will be described below in detail regarding the DMFC using methanol as a typical fuel.
Fuel exhaust gas contains carbon dioxide as a principal component which is generated by the following reaction (see formula 1). The reaction (represented by the formula 1) is the sum of half cell reactions of an oxidation reaction (see formula 2) on an anode and a reduction reaction (see formula 3) on a cathode, and means that 1 mole of CO2 is generated by exchanging six electrons per cell.CH3OH+ 3/2O2→CO2+2H2O  (formula 1)CH3OH+H2O→CO2+6H++6e−  (formula 2) 3/2O2+6H++6e−→3H2O  (formula 3)
FIG. 1 shows a basic configuration of a DMFC system 101. With reference to FIG. 1, the first problem will be described below.
A DMFC body 102 is located substantially at the center of a DMFC system 101. A methanol-containing fuel to be used for electricity generation at the DMFC body 102 is charged into a methanol container 103. Methanol stored in the methanol container 103, which may be 100% purity methanol, is generally an aqueous methanol solution. A necessary amount of methanol is introduced from the methanol container 103 through fuel supply lines 113 and 115 into a fuel mixing tank 108 by fuel supply means 104, such as a bulb or a pump. The methanol can be supplied to a midway of the fuel supply line 115. The fuel supply means 104 is operated when the methanol concentration is equal to or less than a predetermined concentration. These controls are performed by an automatic control mechanism, such as a microcomputer. In the system 101, a controller 109 controls the fuel supply means 104 through a signal cable 125.
When the methanol concentration in the fuel stored in the fuel mixing tank 108 exceeds the upper limit, pure water supply means 107 is operated so that necessary water is supplied from a pure water container 106 through pure water supply lines 114 and 116 to the fuel mixing tank 108 to maintain the methanol concentration in an appropriate range. The pure water can be supplied to the midway of the pure water supply line 116. The controller 109 also controls the pure water supply means 107 through a signal cable 126.
The concentration of the aqueous methanol solution in the fuel mixing tank 108 is adjusted into a predetermined range by the above method. A part of the fuel is sucked up by a fuel circulation pump 112, taken into the fuel circulation line 150, and then supplied to the anode side of the DMFC body 102. At the anode, the methanol is oxidized according to the formula 2. Thereafter, drainage of methanol is returned again to the fuel mixing tank 108 through the fuel circulation line 150.
Methanol concentration measuring means 129 is disposed between the fuel circulation pump 112 and the DMFC body 102, and measures the methanol concentration just before being supplied to the anode of the DMFC body 102. The methanol concentration measuring means 129 can use a sensor for calculating the methanol concentration from measured values, such as a density, a refraction index, and an amount of absorption of infrared light of the fuel. Data on the methanol concentration measured by the methanol concentration measuring means 129 is transferred to the controller 109 through a signal cable 130. The operation of the fuel supply means 104 or the pure water supply means 107 is corrected according to the data on the methanol concentration (feedback control).
Carbon dioxide generated in the oxidation reaction of methanol (formula 2) exists in solution or in the form of minute bubble at the DMFC body 102. The carbon dioxide is moved to the fuel mixing tank 108 through the fuel circulation line 150. In the fuel mixing tank 108, most of the carbon dioxide exists in a vapor phase. When the pressure of the vapor phase is increased, the carbon dioxide is discharged outside the system 101 through exhaust gas lines 127 and 128 and a vapor-liquid separator 110. The vapor-liquid separator 110 may incorporate a catalyst reactor to remove an organic compound, such as methanol contained in a very small amount in the fuel exhaust gas.
Air is supplied to the cathode side of the DMFC body 102 through an air supply line 141 by air supply means 140, such as a fan or a blower. In the cathode, a water-forming reaction proceeds (see formula 3). The exhaust gas after generation of electricity passes through an air discharge line 142 and then is emitted outside the system 101.
The methanol concentration contained in the fuel to be supplied to the anode side of the DMFC body 102 is measured by the methanol concentration measuring means 129. The measured physical amount is normally affected by temperature. For example, the density of the fuel is influenced by thermal expansion of the fuel. Further, the sensitivity of measurement of a measuring device itself in the methanol concentration measuring means 129 is dependent on the temperature.
The fuel temperature is dominated by the temperature of fuel emitted from the DMFC body 102. The temperature of the fuel from the DMFC body 102 is affected by a current value (or output), a change in outside air temperature, or a change in cell resistance over time. In particular, the temperature of the DMFC body 102 transiently varies before and after the change of the output. Therefore, the measurement of the methanol concentration tends to become inaccurate before the temperature becomes stable.
Thus, when an actual temperature of the fuel emitted from the DMFC body 102 varies, the fuel temperature needs to be corrected in response to the variation. The correction over time of the fuel temperature is also a critical technical problem so as to stably manage the concentration.
The second problem to be solved by the present invention is to enable a setting of the optimum fuel concentration in response to a change in required output.
It is known that, in the DMFC, methanol existing in the anode (fuel electrode) penetrates the electrolyte membrane and moves to the cathode (oxidant electrode), and a direct oxidation reaction occurs between oxygen and the methanol on the cathode. This is the so-called methanol crossover. The occurrence of the crossover drastically reduces the potential of the cathode, leading to problematic reduction of the whole cell in voltage between terminals and in output.
As the methanol concentration is decreased, the level of methanol crossover amount can be relatively reduced but the output is also reduced due to the shortage of the fuel under a condition for requiring the high output. When the methanol concentration is high, the output reduction due to the methanol crossover becomes large with respect to the output under a low output condition. That is, the system 101 has a low efficiency of fuel utilization.
For this reason, the operation method which can control the optimum methanol concentration based on the output is more desirable. This is the second problem to be solved by the present invention.
The third problem to be solved by the present invention is to provide an operation method which can autonomously adjust the optimum methanol concentration even when catalytic activity is degraded. This problem arises when the duration of use of the system becomes long and the cell performance is degraded over time.
A membrane electrode assembly (hereinafter referred to as a “MEA”), a gas diffusion layer, and a separator used in the DMFC body 102 have their performance or nature changed gradually by driving over the long time. For example, the activity of the electrode catalyst of the MEA tends to be decreased due to gradual elution or aggregation of catalyst particles over the operating time. The gas diffusion layer may exhibit the flooding phenomenon in which the electrode layer excessively gets wet and shows lower activity due to a reduction of water repellency and drainage capacity. When the superficial nature of the separator is changed, the plugging phenomenon may occur in which formed water remains in the passage and interrupts the flow of air. A cell voltage may be decreased due to an increase in contact resistance between the separator and the gas diffusion layer. These complicated mechanisms gradually reduce a cell output.
Therefore, an adjustment of the fuel concentration is a very effective fuel control technology, which cannot decrease the output as much as possible or can extend the life of the cell as long as possible when the cell performance is degraded. Accordingly, the third problem of the present invention is to provide an operation method which can autonomously adjust the optimum methanol concentration even when the catalytic activity is reduced.